Only Raven can make a new game in the Hexen/Heretic universe, id has pretty much nothing to do with it. They haven't announced a new sequel, they're busy enough with SoF II and Jedi Knight II right now, but it could happen...

We'll along time ago (after Heretic II came out) I recall some pc mag having an interview with raven. They asked where do you see heretic in 3 years. Raven replyed somthing like, "Where done with these types of games, we want to move onto something more realistic". But then again, they could come around and realize...HEXEN 3 WOULD OWN YOU AND IT HAS TO BE A MULTIPLAYER ONLY GAME LIKE Q3A...but thats only my opinion.